Since the advent of humanized antibodies, the therapeutic use of antibodies such as Rituxan® (rituximab), Herceptin® (trastuzumab) and Avastin® (bevacizumab), has revolutionized the fields of medicine, including oncology, the treatment of inflammatory disorders, such as rheumatoid arthritis, and many other indications. In the United States, more than 30 human or humanized antibodies have been approved for clinical use, and more than 600 new antibodies or antibody-like molecules are in various stages of development. Some antibodies have antagonistic function on soluble target molecules such as vascular endothelial growth factor (VEGF) or tumor necrosis factor (TNF), whose actions are part of the pathologic process of a disease. Alternatively, antibodies can bind, block and/or induce destruction of pathologic cells in certain diseases, such as cancer. The main functions of these therapeutic antibodies are binding through the Fab region, and recruitment of effector function via the Fc domain (which also mediates the long circulating half-life of antibodies). One of the major advantages of antibodies compared to small molecule drugs, can be their exquisite specificity. Antibodies can very accurately target selected protein antigens, such as oncogenes, to the exclusion of very similar homologs, allowing for benign safety profiles. Hence, antibodies are well characterized for specific single targeting function.
As the field has progressed, antibody function has been enhanced through creative means of protein engineering, such as to provide higher affinity, longer half-life, and/or better tissue distribution, as well as combination of small and large molecule technologies for increased focus of cell destruction via toxic payload delivery (e.g. antibody-drug conjugates). Another approach to improving antibody function takes advantage of the bivalent binding of the immunoglobulin G (IgG) structure which allows one IgG molecule to bind two antigens. Indeed, in certain applications, there exists good potential for asymmetric antibodies to exert useful functions by simultaneously binding two different target antigens. To address this need, a variety of constructs have been produced to yield a single molecule that can bind two different antigens, allowing for functions never before seen in nature. An example of this bi-specific approach is “blinatumumab” (MT103) which binds the CD3 and CD19 receptors, on T- and B-cells, respectively. This tethering of a cytotoxic T cell to a cancerous B-cell, allows for effective treatment of B-cell leukemia.
However, there remain significant technical difficulties in construction, expression and production of bispecific antibodies. Although bispecific antibodies are regarded as promising therapeutic agents due to their ability to simultaneously bind two different antigens, their utility is limited due to problems with stability and manufacturing complexity.
Various forms of protein engineering have been used to match heterologous heavy chains, plus appropriate pairwise matching of heavy and light chains to efficiently yield a bi-specific IgG. In addition, various bispecific antibody formats, including quadromas, chemical heteroconjugates, recombinant constructs using selected heterodimerization domains and recombinant constructs of minimal size consisting of two minimal antigen-binding sites.
However, all of these efforts have been fraught with difficulty.
Thus, despite efforts directed toward the development of bispecific therapeutic antibodies, there remains a great need for developing more efficient platforms that can lead to more efficient and flexible production of bi- and multispecific antibodies, thereby shortening the timeline between discovery and clinical introduction of such therapeutics and enabling the design and production of new types of antibody formats with multiple specificities and/or valencies.